A polyamide resin where the dicarboxylic acid component is oxalic acid is called a polyoxamide resin and is known to have a high melting point and a low saturated water absorption compared with other polyamide resins having the same amino group concentration (see, Japanese Unexamined Patent Publication (Kokai) No. 2006-57033).
Polyoxamide resins using various diamines as the diamine component have heretofore been proposed. The polyoxamide resin is obtained by the polycondensation of oxalic acid or an oxalic acid diester with an aliphatic or aromatic diamine. However, in the case of using oxalic acid as the monomer, the oxalic acid itself thermally decomposes above 180° C. and the end of oxalic acid is also end-capped during the thermal decomposition. For this reason, there is not known a synthesis example where a polyoxamide resin having a high molecular weight is obtained.
On the other hand, there is also known a method for producing a polyoxamide resin where an oxalic acid diester such as dialkyl oxalate is used as a monomer, and polyoxamide resins by polycondensation with various diamines have been proposed. For example, a large number of polyoxamide resins such as polyoxamide resin using 1,10-decanediamine, 1,9-nonanedimaine or 1,8-octanediamine as the diamine component (see, Japanese Unexamined Patent Publication (Kohyo) No. 5-506466) and polyoxamide resin using 1,6-hexanediamine as the diamine component (see, S. W. Shalaby., J. Polym. Sci., 11, 1 (1973)) have been proposed.
However, for all known polyoxamide resins, the production requires a two-step polymerization process, that is, a pre-polycondensation step of performing a polycondensation reaction in a solvent such as toluene to obtain a prepolymer, and a post-polycondensation step of distilling off the solvent from the obtained prepolymer and then performing melt polymerization or solid phase polymerization to obtain a polymer. Accordingly, the production of a poly-oxamide resin has a problem that a large amount of energy and time is required for distilling off the solvent, which is not suitable for industrial production.